The invention relates to an apparatus for controlling an acousto-optical component in order to influence light passing through, in particular in order to influence the illumination light and/or the detection light in the beam path of a microscope, preferably a confocal laser scanning microscope, having a radio-frequency generator for supplying the acousto-optical component with a radio frequency. Further, the invention relates to a corresponding method as well as uses and applications of both the apparatus and the method, respectively.
Basically, this is about the control of acousto-optical components in order to influence light passing through. Such components typically comprise an acousto-optical crystal on which an electric transducer is provided. Typically, the transducer comprises a piezoelectric material sandwiched by electrodes. By electrically applying radio frequencies, which are usually in the range of 30 MHz and 800 MHz, to both electrodes the piezoelectric material is oscillated so that an acoustic wave (sound wave) is created which, as a result of the arrangement of the transducer, passes through the crystal. After passing through the optical interaction area the sound wave is usually absorbed or reflected on the opposite crystal side. Acousto-optical crystals as used with the acousto-optical components in question here are characterized in that the created sound wave changes the optical property of the crystal, a diffraction grating or a comparable optically active structure, for example in the form of a hologram, being induced by the sound. Light passing through the crystal is diffracted at the diffraction grating created in this way, the light being directed in different diffraction orders or diffraction directions.
In the acousto-optical components in question here one distinguishes between components which influence the entire incident light more or less independent of the wavelength (e.g. AOM, AOD and frequency shifter) and components which, for example depending on the radio frequencies, selectively act on individual wavelengths (e.g. AOTFs).
Often, the acousto-optical components are comprised of birefringent crystals, such as tellurium dioxide, the position of the crystal axis relative to the plane of incidence of the light and its polarization determining the optical properties of the acousto-optical component.
In specific applications, the light uninfluenced by the diffraction, the light diffracted in different diffraction orders, or both the uninfluenced as well as the diffracted light are optionally used.
In the acousto-optical components known from practice, the radio frequency (RF) is supplied to the acousto-optical component usually via a coaxial cable. Thereat, an impedance matching takes place on an electronic circuit board, wherein care has to be taken that no RF reflections occur. As much RF power as possible should reach the crystal which usually has an impedance different from that of the RF cable. From the electronic circuit board, the radio frequency is forwarded to the transducer on the crystal, where the acoustic wave is created.
In the past, the acousto-optical components in question here, mainly in the case of AOTFs, were mostly used to set and control light intensities. Recently, there is a need to use respective components for “cutting out” specific portions of the light from a more or less spectrally broadband light. With regard thereto, reference is made to DE 101 15 488 A1 by way of example only.
The acousto-optical components in question here serve within the above-mentioned uses mainly to cut out specific spectral portions of a continuous or broadband light source for illumination purposes. By way of example only, reference is made to the use in connection with white light lasers, broadband lasers, ultrashort pulse lasers, superluminescent LEDs or other superluminescent light sources, ASE light sources, bulbs, point source LEDs and others LEDs, sunlight or starlight etc. The optical components also serve to cut out specific spectral light portions for detection purposes, for example for use in programmable spectral filters. Also the use of the acousto-optical component within a programmable beam splitter (AOBS) is of importance. Further, it is known from practice that the acousto-optical components in question here change their behavior over the temperature profile, this being mainly attributed to a change in the velocity of sound in the crystal. If one wishes to use the acousto-optical component at changing temperatures, a compensation of the behavior caused by the change in temperature is necessary. Corresponding compensation methods are already known. These methods suggest to heat or to cool the crystal which is exposed to the temperature fluctuations in order to cause a temperature stabilization at the crystal. For this, a special temperature control is provided. In this respect, reference is also made to EP 0 834 762 A2, according to which some sort of dummy radio frequency is provided which is fed in whenever the actual radio frequency is turned off so that the same heat can always be deposited in the crystal via a heating system.
As an alternative to the above-mentioned method, the radio frequency is adjusted according to a measured change in temperature as specified in DE 198 27 140 C2. However, up to now one assumed that the relevant compensation parameter as well as the radio frequency itself, which is required for operating the acousto-optical component, depends on numerous parameters such as on the wavelength of the light to be diffracted, on the angle of incidence of the light in the crystal, on the mounting conditions of the crystal etc. Therefore, up to now, the amount of the change in frequency has been determined iteratively experimentally, or one compiled tables for the compensation parameters depending on the wavelength and on instrumental conditions. In this connection, it has been necessary to individually determine the compensation parameters for each individual device. In this respect, reference is made in particular to section of DE 198 27 140 C2.
The effort to be made within the scope of error compensation according to the printed prior art is huge since for each laser wavelength used and possibly for each system used special correction parameters have to be stored and handled. On top of that, it is required to supply the drive electronics with information in order to define which specific laser wavelength and which experimental parameters are present so that the compensation parameter related thereto can be inserted. Consequently, according to the printed prior art the temperature compensation cannot be directly performed by the radio-frequency generator but rather has to be supported or even performed entirely by a higher operating level since for temperature compensation all system information required has to be liquid. This is opposed to a simple operability of the system as well as a fast temperature stabilization on small time scales. Thus, it is, for example, necessary in a confocal microscope to provide a higher software level which has information available as to which laser wavelengths are to be diffracted from the crystal, which provide the necessary compensation parameters for the radio-frequency generator so that the radio-frequency generator can perform the frequency control correctly. Accordingly, the temperature compensation as specified in the printed prior art is not performed by the radio-frequency generator but by the computer controlling the same, which computer—mostly in an unchangeable manner—provides the radio-frequency generator with already temperature-compensated radio frequency nominal values. This results in an enormous complexity and error-proneness of the entire system.